Chatting on Facebook Messenger right now requires one to visit the Facebook website, which can easily get lost amidst a bunch of other tabs that you have open. With a native application, it will be possible to use Facebook Messenger to chat with your friends without the need to open your web browser.
The app is believed to be an official client rather than an unofficial product. It uses the Messenger logo as the icon in the Dock and is named Messenger in the menu bar.
There are quite a number of unofficial third-party Facebook Messenger app for both Windows and OS X. If anything, these apps do show that a demand for a native Messenger client for desktop is definitely there.
At the moment, it is unclear when, or if, Facebook plans on releasing its Messenger app for OS X.
[Source]